For anyone else who is as frustrated by the uncertainty involved in the CYA test, here are some images to help reassure, at least. Obviously, everyone's vision is different and images are only so good, but comparison helps. When they say keep the sun behind you, it's more than "make sure it's light out" they mean your eyes should not be able to adjust to the shade and no light should hit the water directly from the side. I purchased 50 ppm reference water and it seems the only way to get an accurate reading is midday sun with no clouds and the sample held in the shadow of your head.
Notice mainly that you can still perceive the dark spot, it just looks like it's becoming really blurry at the correct level, and if the whole tube is the same color you are likely past it:
Notice mainly that you can still perceive the dark spot, it just looks like it's becoming really blurry at the correct level, and if the whole tube is the same color you are likely past it:
